This invention relates to plasticized polyvinyl butyral (PVB) sheet and more particularly to a method of regulating the surface roughness of such a sheet.
Plasticized PVB sheet is very well known as an intermediate for use with glass or plastic panels (hereinafter called "laminating panels") in laminated safety glass assemblies.
It is further known, as typically disclosed in U.S. Pat. No. 4,305,549, that the surface of the extruded sheet should have a certain degree of roughness to facilitate deairing, i.e. the evacuation of air from the interface of a PVB sheet and laminating panel during preparation of a laminate. More specifically, channels between the smooth surface of the laminating panel and the extremities of minute valleys in the face of the rough surface of the opposing, contiguous PVB sheet permit air between the two members to escape when pressure or vacuum is applied to the interface. However, since deairing is carried out differently by various laminators, different levels of sheet roughness may be required. For example if roughness is too low for a particular deairing process the air evacuation channels may close down prematurely and the edges of the sheet will seal to the laminating panel before deairing is complete. On the other hand, if the channels are too deep, it may be difficult to remove all the air before the sheet surface flattens against the laminating panel.
Different sheet roughness levels have been provided in the past by changing the surface configuration of die member(s) in the extrusion operation, but this is costly since it requires shutting down the extrusion process to change such die members. It would be desirable to provide a more tractable way to alter PVB sheet roughness.